In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
Various techniques have been used for electronically controlling the oncoming and off-going clutches during a power-on downshift. For example, the U.S. Pat. Nos. 5,029,494 and 5,070,747 to Lentz et al. disclose power-on downshift controls in which the off-going clutch is controllably released in an established sequence to allow the engine to accelerate the transmission input shaft to a post-shift or synchronization speed, whereafter the shift is completed by engaging the on-coming clutch and dis-engaging the off-going clutch. Similar control techniques are also described in the U.S. Pat. No. 4,653,351 to Downs et al., and the U.S. Pat. Nos. 4,796,490 and 5,079,970 to Butts et al. Additionally, it is known that the shift quality can be enhanced by momentarily reducing the engine torque as the input speed approaches the synchronization speed. For example, in Butts et al. U.S. Pat. No. 5,079,970, the engine spark timing is retarded when the slip of the on-coming clutch drops to a predefined value; the spark retard reduces the engine output torque by a predefined amount, and is immediately ramped out at an open-loop ramp rate to achieve a smooth torque transient upon completion of the shift.
While the known engine torque controls have the capability of enhancing downshift quality, control parameters such as the timing and quantity of torque reduction have been only generally defined, so that the potential enhancements are fully achieved only occasionally. Also, it has been found that torque controls not uniquely suited to a given set of shift conditions can actually degrade the shift quality. Accordingly, what is needed is an engine torque control that is uniquely suited to the particular shift conditions, and that can respond to and correct aberrant conditions occasioned by errors in the engine torque and clutch controls.